Hey guys. New too all this.
I just picked up a 64 from Alberta. No salt there.
Its in good shape.
I have searched this forum, but Im trying to figure something out.
Mine has the I6 with 4 speed.
These 4 speed trannies, I have heard good and bad.
Pretty much first gear is useless but very durable trannies.
But, how much HP can they take.
I plan on putting a V8 in there and would be around 400 HP.
Is this tranny capable of handling that, or should I look at a different tranny.

With the SM420, SM435 and SM465 four speed transmissions, 1st gear is not intended to be used in everyday driving. You start out in 2nd gear, so you basically have a 3 speed trans. First gear is used when low speed crawling or pulling power is needed. I see no reason a SM420 won't hold up behind a 400hp V8

For a frequent driver I have to agree that a 5 speed manual or a 4 speed auto OD trans makes more sense.

Captain is right about the durability of the trans.
I went through all this decisioneering for my Suburban. Like your GMC, I had a Dana 44 in mine, which gives a lot more options.
I made a chart that showed the gear splits with the Granny 4spd (yours), and a 4spd OD manual trans (since I had one and it was a bolt-in).
I figured effective ratios with the trans gears and different rear end gears, with an eye to final drive ratio at highway speeds.

With the Granny 4-speed, you could get 2.90 or 3.08 or 3.23 gears in the rear and still leave the line, with the 4spd OD I have (.72:1 final drive), I went with 3.42 gears.

I like jack-rabbit starting and 3.42 gears are still enough to pull a light trailer, so that was why I went with my choice.

5-speeds are a great choice too, but I didn't have one, and I don't know what's required to put them in our trucks as far as bellhousings and new cross members. The 4spd OD was a bolt-in for my '62 torsion-bar/X-frame truck with the bellhousing engine mounts.